1. Technical Field of the Invention
The present invention pertains in general to a method for distinguishing between control signaling and user data, and more particularly, to a method for distinguishing between in-band signaling and user data in a communication channel of a communication system.
2. Description of Related Art
In a given communication network conforming to any one of a number of communication network protocols, there are two broadly defined types of transmissions. The first type is referred to as "user data." User data comprises communication messages which are the intended subject of the communication network and include speech, facsimile data, and electronic data, among others. The second type of transmission is control signaling which is communicated via the communication network in order to control the functioning of the network. Control signaling is a part of the overhead necessary to implement the communication system. Typically, communication network protocols are designed to accommodate both types of transmissions over the same physical communication channel while at the same time maintaining separation between them using different logical channels.
In the Global System for Mobile communications (GSM) standard, user data is transmitted over a traffic channel while a portion of the control signaling is transmitted over a different logical channel called a Slow Associated Control Channel (SACCH). Situations occur, however, where the Slow Associated Control Channel does not have the bandwidth necessary to facilitate transmission of all the control information. Examples of when this might occur include DTMF signaling, providing supplementary services, and handoff directives.
In the case of a handoff directive, for example, a telephone conversation needs to be handed over from one base station to another. This handoff requires a large amount of control information to suddenly be communicated over the communication network. The Slow Associated Control Channel is incapable of transmitting the requisite large amount of information and therefore a Fast Associated Control Channel (FACCH) implemented on the logical channel comprising the traffic channel is used to transmit the signaling information necessary to effectuate the handover in place of a 20 millisecond segment of speech. This substitution of control signaling for user data is commonly referred to as in-band signaling. While in-band signaling does not seriously degrade the quality of the user data transmission, it places additional overhead on the communication system.
Since the substitution of control signaling for user data occurs at random time periods from the perspective of the receiver, a method to distinguish between user data and in-band signaling is required. In the GSM standard, user data is distinguished from in-band signaling through the use of a "stealing flag" bit, which is associated with each 57 bits of encrypted user data. Two 57 bits of encrypted data along with two stealing flag bits and other control bits form what is referred to as a normal burst. Four such normal bursts taken together form a Fast Associated Control Channel. Although this solution in GSM is relatively simple to implement, the bits which are used for stealing flags are lost to other uses and become fixed overhead of the overall communication system, thereby decreasing the bandwidth available for user data.
There is a need, therefore, for a method of distinguishing between in-band signaling and user data which does not unnecessarily waste communication bits.